1. Field of the Invention
The present invention relates to a method for dimensioning link bandwidth for elastic-data traffic. More particularly, the invention relates to the dimensioning of bandwidth for elastic-data traffic in a high-speed communications network in which the number of connections to be supported on a particular link is determined. Moreover, given that the bandwidth of the link is determined, the invention relates to a method for determining the maximum number of connections that can be supported on the link.
2. Description of the Related Art
The use of data networks for the reliable high speed transport of information, such as text, voice, video, etc., over wide areas in digital format via a variety of media including optical fibers is bringing major changes to network services and network architecture/infrastructure design. Many new services having diverse characteristics and bandwidth requirements are now feasible.
FIG. 1 illustrates a small portion of a typical high speed network 10, comprising switching nodes 11 which are connected by bandwidth data links 12. Inputs to the network 10 are received by a plurality of user end-systems 13 which are connected to network 10 by corresponding links 14. The inputs may be of any form, but are conveniently formatted at the user end-systems 13 into packets for transmission to other user end-systems 13. Data is transmitted between user end-systems 13 as elastic-data traffic. A link 12, 14 may be the entire bandwidth available to a transmission path between network nodes or between a user end-system 13 and a network node 11, or may be a portion of the bandwidth of the transmission path. Inherent in the transfer of elastic-data from one user end-system 13 to another is that the rate of transfer is directly dependent on the constraints imposed by the network 10 as well as the constraints imposed by equipment employed at the user end-system 13. For example, some factors which may constrain the transport of data include the protocols used to transmit the data, the amount of congestion in a given link 12, 14, the bandwidth of the link 12, 14 and how many connections are simultaneously being transported over the link 12, 14.
In designing a high speed network, an important step is the dimensioning or capacity assignment of each link within the network. The dimensioning step typically occurs once the location of the nodes (e.g., the city in which a particular node will reside) and the connectivity between nodes (e.g., which nodes will be interconnected) is determined.
Elastic-data traffic adapts to time-varying available bandwidth over a communications link using a feedback control scheme. Examples of such schemes are the Transmission Control Protocol (TCP) over the Internet or an Available Bit Rate (ABR) transfer capability in Asynchronous Transfer Mode (ATM) networks. Typical elastic-data applications are the transfer of files supporting e-mail or the transfer of files over the world wide web. A file is a digital object that encodes information which, for example, may be a document, a computer program, data-base records, or an audio or video recording. A file resides in some memory or storage device associated with the user's end-system. The problem of dimensioning bandwidth for elastic-data applications can be viewed as a variation of the well-known "capacity assignment problem" described in the literature on design of computer networks. For example, see D. Bertsekas et al., Data Networks 2nd Edition, Prentice Hall, Englewood Cliffs, N.J., 1992.
Prior art approaches to the capacity assignment problem calculate load based on a flow of packets whose characteristics are assumed to be exogenous (independent) of the state of the network including the presence of other flows. The problem also considers a performance criterion in the form of delay of packets. The outcome of the capacity assignment problem is flawed in that the calculations do not take into consideration the dependent character of the packet flows of elastic data traffic, or that the performance criterion of most interest to the user is the transfer delay of the entire file.
Dimensioning bandwidth for elastic-data traffic can also be compared to conventional traffic dimensioning for telephone networks. Conventional dimensioning of telephone circuits uses the well-known Erlang blocking formula, while a generalized Erlang blocking model is used for multi-rate circuits. Recursive solutions associated with conventional dimensioning of telephone circuits are disclosed by J. S. Kaufman, Blocking in a shared resource environment, IEEE Transactions on Communications, Vol. 29, 1981, pp. 1474-1481; and by J. W. Roberts, G. Pujolle (ed.), A service system with heterogenous user requirements, Performance of data communications systems and their applications, North Holland, 1981, pp. 432-431. Asymptotic approximations for conventionally dimensioning telephone circuits are disclosed by Y. Kogan and Michael Shenfild, "Asymptotic solutions of generalized multiclass Engset model," in The Fundamental Role Teletraffic in the Evolution of Telecommunications Networks, Proc. of 14.sup.th Inter. Teletraffic Congress, J. Labetoule, J. W. Roberts Eds., Elsevier, 1994, pp. 1239-1249, and by D. Mitra et al., Erlang capacity and uniform approximations for shared unbuffered resources, IEEE/ACM Transactions on Networking, Vol. 2, 1994, pp. 581-587. Both recursive and asymptotic approaches for dimensioning telephone networks assume constant-rate connections, which is natural for traditional circuits.
Recently, the concept of effective bandwidth has extended the applicability of the conventional techniques used for dimensioning telephone circuits to variable-rate connections by using the concept of a constant "effective" rate. For recent summaries of conventional techniques using constant effective rate concepts, see, for example, C.-S. Chang et al., Effective bandwidth in high-speed digital networks, IEEE Journal on Selected Areas of Communications, Vol. 13, August 1995, pp. 1091-1100; G. de Veciana et al., Resource management in wide-area ATM networks using effective bandwidths, IEEE Journal on Selected Areas of Communications, Vol. 13, August 1995, pp. 1081-1090; and, F. P. Kelly, Notes on effective bandwidths, Stochastic Networks, Claredon Press, Oxford, 1996, pp. 141-168.
The concept of assigning an effective bandwidth is reasonable for specific parameter regions of classes of traffic, such as packet voice, packet video, frame relay, and Statistical Bit Rate (SBR) service in ATM networks. Nevertheless, elastic-data traffic does not have an inherent transmission rate because the traffic adapts to available bandwidth. Consequently, the concept of an effective bandwidth for elastic-data traffic seems dubious. Further, the conventional performance criterion of a blocking probability of a new connection request is irrelevant for best-effort type services if, generally speaking, all connection requests are granted, or if a request is not even made prior to the transmission of the user data.